In simple advice, start your own company first if you can figure out the financial support, because once you start working for another company you will be too busy and also you will have the fear to start your own company because it will drain all your time and money. The fear of difficulty in getting the documents done on time, the fear of losing so much money and the fear of not having enough time for your work will stop you from starting your own business. 
However, if you start your own company first then you will have plenty of time to focus on your own business, you will also have no fear in doing all the documentations required. The only fear will be losing money in business, but if you are the only employee of your company then you can break-even fast to avoid spending more money. I have met many people who works for big companies, and only option they have is investing their money on another entrepreneur because they said they don't have time to be involved in the business. 
How can you manage your business while working for another company?
The best way to manage business while working for another company is by having the right manager for your company and outsourcing your legal work such as accounting to another audit companies. This will reduce your time and help you to focus on more important things. 

The cost is something that we all hate, but without the cost it is hard to gain revenue. The investors love entrepreneurs that knows how to reduce the cost so that there is no additional cost on the investors. However, it is very hard to find an entrepreneurs that knows how to benefit from reducing cost. The impact of reduction in cost can also increase the revenue, that is if the entrepreneurs have full awareness of the business that they are in. The example of increasing revenue is when you start the production within your company. For example if a coffee shop can roast their own coffee to reduce the cost of purchasing coffee, and at the same time increase revenue from selling packaged coffee. Another example is if the restaurants can have self service, it will help to reduce the cost of wages and as well as it is unique way of having fast food restaurants for the customers. Having said this, the restaurants can have an increase in revenue and reduction in cost by eliminating dishwashers and changing for takeaways. The dishwasher is a big cost to the restaurants, the dishwasher cost about $3000 and it it needs to be attached to drainage and to water supply. The cost of detergent is also reasonably high for commercial dishwasher, and the maintenance of hygiene is another problem which can have a high cost. All these can be eliminated by simply self service takeaway packs. These are the simple examples of reducing the cost but at the same time increasing revenue. Every business is different and it is important for managers to make proper analysis that requires outside the box thinking.